Once I was told that you can identify if your mobile phone is being tracked by someone (e.g. police), by calling placing a specific prefix to the number. This prefix in my country is normally reserved for for landline phones, not for mobiles.
I tried it for many mobiles, including mine, and all I get is just a "beep", not wrong subscriber message, nothing.
Recently one friend of mine has some minor law problems, and suddenly if I call his number including the prefix, it not only calls normally, but also a lady picked up the line, asking who I am. Every time we called her, when she realized that we were not the one she was wanting to call her, she was hanging up, without explaining who she is.
I believe that my friend's calls are intercepted by someone, and that in fact I talked to the attacker. This behavior is constant no matter where my friend is (so no over-the-air man-in-the-middle attack), and with any mobile phone (so no spying app, or hacked electronics).
Who can we identify the attacker, and/or make sure that my friend will get out of all these?